The present invention relates to an electromagnetic coupling device used in an electromagnetic clutch for transmitting power, an electromagnetic brake for braking power, or the like. Specifically, the present invention is concerned with an electromagnetic clutch configured so that divided armatures are attracted magnetically in a stepwise manner, thus providing a progressive coupling between a magnetic flux producing member and armatures.
Electromagnetic clutches or electromagnetic brakes have been widely used for computer peripheral equipment, copy machines facsimiles devices and the like as their quality and performance have been recently improved. in addition, in an electromagnetic clutch or the like for a car cooler which is mounted on an automotive vehicle and transmits engine power to a compressor, it is required to transmit engine torque suitably damped for preventing a load to the the engine from being suddenly changed.
An electromagnetic clutch as disclosed in Japanese Utility Model Publication No. 47-22293 is typical of an electromagnetic clutch having such a damping function. This electromagnetic clutch is provided with an electromagnet comprising an excitation coil and a yoke for clamping the excitation coil disposed within a drive rotor to magnetically attract armatures comprising external and internal poles and supported by hubs by means of two spring plates having different spring constants.
Conventional electromagnetic coupling devices of this kind have the following drawback; it is impossible to exert a necessary attraction force to an armature first attracted by an magnetomotive force produced by the excitation coil. As a result, a torque rising characteristic in the armature first attracted is degraded. For this reason, it is unable to obtain a damping action for providing a desired progressive coupling between the drive rotor and the armatures.
Another conventional electromagnetic clutch having the above-mentioned damping function is disclosed in Japanese Utility Model Application Laid-open No. 56-44227. This electromagnetic clutch is configured such that there are formed different lengths of magnetic gaps between a friction surface of a rotor to which a V-shaped pulley is fastened and radially positioned inner and outer armatures, whereby the outer armature is forcedly moved by utilizing projections formed at the respective armatures when the inner armature is magnetically attracted. In this instance, armatures have thicknesses different from each other in order to obtain different lengths of the magnetic gaps.
However, since this electromagnetic clutch is configured so as to vary attraction forces exerted on the armatures only by adjusting magnetic gaps, it is difficult to overcome the drawbacks encountered in the first-mentioned conventional electromagnetic clutch. Namely, this electromagnetic clutch fails to suitably adjust an amount of magnetic flux toward localized armatures, although such an adjustment is necessarily required in order to attract them in a stepwise manner. Accordingly, the last-mentioned electromagnetic clutch is also not practically acceptable.